This quote is found at the very beginning of the story, introducing Montag's character, the nature of the firemen, and depicting the grotesque pleasure that Montag initially finds in his job.
Much of this opening passage is devoted to floral, extravagant language that attempts to paint what some might consider an ugly and reprehensible act (burning books) as something beautiful and poetic. This is central to the reader's understanding of the firemen, their society, and the change in character that Montag experiences. We must share, or at least see, the perception of burning books as an attractive thing, and detach ourselves from the meaning which has been connecting to that act in our own society. In this society, burning books is a good thing, which is at least part of the reason why it can be seen as beautiful.
The specific quote suggests that burning is like music; Montag is the conductor, leading a performance. Saying "fires are like symphonies" might suggest that they are complex, "high-class", or intrinsically beautiful, and that Montag's job is both a spectacle and a refined craft.